Chapter 16 & 17 - Pracatical applications of immunology & Immune Disorders
There are many practical applications that one can use in the diagnostic immunology laboratory with respect to disease detection and disease monitoring. Many of the same processes that occurr in the human body, such as Ag-Ab interactions, can be utilized in a laboratory setting to augment disease conditions. The following 3 laboratory based detection methods are examples of very common diagnostic tools used in clinical settings, public health labs, and in research settings. Be aware that there are MANY more of these types of detection methods. I have chosen to describe the following 3 because of there "basic" principles behind each technique and due to their popularity with respect to high sensitivity and specificity.
1) Agglutination reactions occurr when an Ab and Ag are specific for each other and a "lock and key" lattice of Ab-Ag forms to allow for an observable agglutination reaction. An example is the Rapid Plasma Reagin (RPR) kit used for the detection/screening of syphillis caused by the bacterium, Treponema pallidum. The IgM antibody is often referred to as early Ab or the 1st Ab to appear in an infection. It is also known as the Ab of agglutination because of it's molecular make-up. IgM is typically arranged as a pentamer (5 Ab's bound together which allows for 10 sites of Ag attachment). An IgM pentamer (see figure in text) allows for many Ag's to bind and form a complex lattice that will become insoluble in solution and be visible to the naked eye. Thus, one can use specific Ab for syphillis to bind to specific Treponema pallidum Ag's and form a complex (this is typically called a "reactive"). The test is easy and low cost....because of these features, it makes an RPR test a useful and cheap screening test for this disease.
2) Fluorescent Antibody (FA) assays are one of the most common kits available to diagnostic labs for the detection of a variety of microbes. A kit will simply have an Ab that has been "labeled" with a fluorochrome such at FITC. The FITC will fluoresce when exposed to a uv light source on a microscope and "light up" as apple green fluorescence. If the reaction on a slide is not specific (ie. the Ag doesn't match the labeled Ab) then there is no fluorescence. See text for illustrations of this test.
3) Enzyme Linked Immunoabsorbent Assays (ELISA) are also a very common kit available for diagnostic labs. The HIV screening test is an ElISA based test. The kit will have a specific Ab for HIV in a plastic coated well. A technician will then add serum from a patient. If the serum has HIV Ag, it will bind to the Ab in the plastic coated well. This step is followed by another Ab that is labeled with an enzyme. Then a substrate is added to the "sandwich" ELISA. If the labeled enzyme has attached in the previous step, the substrate will fit into the enzymes active site and a colorimetric reaction will take place. So, a typical color like yellow will be produced with a positive test. See the text for illustrations of this test.
Immunological disorders are caused when the immune system malfunctions, producing either an inappropriate or inadequate immune response.
Hypersensitivity - a misdirected response in which either Ab's or T cells cause damage.
While there are 4 types of hypersensitivities, I have chosen to discuss only those that are related to responses to microbes. You will probably learn about the others in an Anatomy & Physiology course.
A. Types of Hypersensitivity:
1. Type III (Immune-complex ) hypersensitivity - antibody mediated; IgM & IgG antibodies react with a person's own antigens - bind to antigens that are free in circulation (in Type II reactions, Ab binds to Ag on tissues & cells); Ab-Ag binding creates complexes that remain soluble in body fluids; complexes can lodge in capillaries, activating complement & provoking an inflammatory response & attracting phagocytes; Ex:
a. viral hepatitis - chronic infection from long-term exposure to microbial antigens
b. farmer's lung - contracted by people who continually inhale certain antigens from molds, plants, or animals
c. serum sickness - occurs when proteins from animal serum are used in medical therapy; ex. horse antiserum is used in the treatment of venomous snake bites; patients receive an infusion of horse antibodies to bind to the snake venom antigen; the patients may produce antibodies against the horse antibodies, forming large complexes.
2. Type IV (Cell Mediated or Delayed) hypersensitivity - does not involve Ab's; a T cell encounters an Ag that matches its Ag receptors; this stimulates the T cell to divide; when sensitized or activated T cells encounter the Ag again they release interleukins that stimulate macrophages and initiate inflammation, causing tissue damage. Ex.:
a. organ transplants (tissue grafts); immunosuppressive drugs are used; cyclosporine is an improvement - a product of a fungus, it interferes with T cell function, but not with B cell function.
b. tuberculosis, leprosy - damage is caused by a granulomatous reaction (when a macrophage cannot completely destroy microbial Ag, it persists within the cell; these persistent Ag's cause T cells to release interleukins that stimulate the production of a granuloma, a nodule of activated macrophages; the continuing inflammation that goes on within & around the granulomas displaces & destroys normal cells).
B. Immunodeficiency Disorders
Immunodeficiency - failure to mount an adequate immune response
1. Congenital - inherited or develop before birth & usually appear early in life; Ex.:
a. severe combined immunodeficiencies (SCID) - B & T cell immunity are disabled.
2. Acquired - occur later in life; caused by infection, cancer, or the side effects of immunosuppressive medication (ex. steroids); Ex.:
a. AIDS - acquired immunodeficiency syndrome
C. Cancer & the Immune System
Cells become cancerous when they under transformation, a process leading to uncontrolled division that produces large numbers of undifferentiated cells. These primitive, rapidly dividing cells form large growths called tumors that crowd and eventually kill normal neighbor cells. Transformed cells often express new surface antigens that are not found on normal cells & mark the cancer cells for destruction by T cells, NK cells, & macrophages.
Cell transformation occurs frequently, but the immune system eliminates most malignant cells before they cause cancer. AIDS patients are at an increase risk for cancer (ex. Karposi's sarcoma) because they lack the white blood cells to kill off the malignant cells. As immune defenses weaken with age, the elderly are also at an increased risk for cancer.
Many anticancer drugs are highly toxic because they are nonspecific (nonspecific tumor destruction) - they kill normal rapidly dividing cells as well as cancer cells (ex. hair cells, intestinal cells, bone marrow stem cells). Cancer immunotherapy would be more effective. If we can determine unique antigens on the surface of cancer cells, then we can target only those cells for destruction.